Offshore pipelines, for example gas and or oil pipelines, are laid by vessels such as water crafts, barges, semi-submersibles or the like. Such pipelaying vessels are well known in the art. Examples in the art of such pipelaying vessels include U.S. Pat. No. 3,967,461, U.S. Pat. No. 4,257,718, U.S. Pat. No. 5,044,825 and U.S. Pat. No. 5,823,712.
Typically, the pipeline is assembled in a firing line (the main production line) composed by a variety of operating stations, such as welding stations and testing stations. The pipeline is extended by joining pipe sections to the end of the pipeline. Such pipe sections (typically called “pipe strings”) are formed of a variety of single length pipe sections (typically called “bars”), which are preassembled either in on-shore yards or directly on board the laying vessel. Such bars typically have a length of 12 meters long and may be formed into pipe strings of 24 meters (a double joint or 2J), 36 meters (a triple joint or 3J), or 48 meters (quadruple joint or 4J). The present invention concerns the case where such pipe strings are preassembled on board the vessel. Such a process is typically referred to as prefabrication. Prefabrication operations carried out on the vessel may include any or all of the following:                Beveling of the ends of the single length pipe sections (bars) for subsequent joining;        Welding between adjacent bars in a pipe string to be prefabricated;        Performance of non-destructive testing on the intermediate weld so formed;        Field joint coating of the intermediate welds so formed (i.e. partially or completely restoring the pipe coating that needs to be removed so as to facilitate welding together of the single bars); and        Beveling of the ends of the single bars, which eventually form the endings of the realised pipe string (joined pipe section).        
Pipe strings prepared in this way may be temporarily stored in an area on the vessel which is used to feed the main production line (the “firing line”). In order to avoid unnecessary downtime, such a storage area may be arranged to hold a predetermined minimum number of pipe strings so as to provide a reliable supply of pipe strings for the firing line. The storage may also be fed by more than one prefabrication system. It will be appreciated that the operating stations in the firing line are arranged in series and are typically separated by a distance equal to the length of the pipe string.
The operating stations in the prefabrication system typically work in parallel and are dedicated to different operations, for example welding (first, subsequent and finishing passes), monitoring (NDT) and restoring of the connecting zone (Field Joint Coating including filling). Typically, there are from two to five welding stations, one NDT station and at least one, perhaps more, FJC and filling stations. Every joint is therefore worked sequentially in several stations. At the end of the cycle, the pipeline being laid is extended by a length of one pipe string. The cycle time is dictated by the slowest station (the critical station). The function of each workstation is usually decided by dividing the tasks between the stations in the most efficient way (each station being assigned one or more operations). In some cases (for example in the case of a multiple bar joint, for example the double joint), there are intermediate stations in the firing line which are arranged to perform only FJC or filling operations on the intermediate welding (realized offline in the prefabrication system). Whether during the prefabrication or during the manufacturing in firing line, welding operations are typically the most likely to cause delay in the pipelaying process. For example, certain welds may require repair or complete elimination (cutting) and remaking operations. Said operations can adversely affect the production cycle time.
The present invention seeks to improve prefabrication efficiency. The present invention alternatively, or additionally, seeks to provide a prefabrication method or vessel, for use in the field of narrow mono-hull vessels, which have the advantage of facilitating the laying of pipelines in different conditions with relatively high productivity. Alternatively or additionally, the present arrangement seeks to provide a more flexible means of laying pipeline than previously proposed in the art. The present invention alternatively, or additionally, seeks to provide a method or vessel for laying pipelines with a large diameter, preferably in deep water.